Yes, how long since repotting this? A lot of people let Brassavolas dry completely between waterings. If you do this the medium won't mold until it is very old.
__________________ May the bridges I've burned light my way.
New purchase, never repotted. I tried to let it dry out completely, but I have a difficult time determining when loose media is dry below the surface. I will repot with new media then.
Brassavolas tolerate a lot more drying than most other orchids. People successfully use large bark, wine corks, scoria or cinder chunks and LECA for medium.
If you have higher humidity they also do well mounted, or bare-root dangling from a wire. They need more frequent watering this way. Use the Search function in the top menu to see how people here have done it.
__________________ May the bridges I've burned light my way.